What is the distinction between a series and a study?
By study, we mean a one-time data collection, a single year, or a collection within a series.
For example, the National Survey on Drug Use and Health (NSDUH) has been conducted annually since 1990, and every 2-3 years prior to that (as far back as 1979). When we talk about all the NSDUH surveys, we refer to the series; when we talk about a particular year, we refer to a study.
When we divide SAMHDA data into "Series" and "Other Studies," we're making a distinction between surveys repeated over time, and surveys that were a one-time event.